Composition useful for preparation of dense neodymium stabilised β-silicon nitride-α-SiAlON composite

ABSTRACT

The present invention consists of a synergistic mixture of Si 3 N 4 , Al 2 O 3 , AlN, SiO 2  and Nd 2 O 3 . The cost effective synergistic composition is useful for the preparation of dense neodymium stabilised β-Si 3 N 4 -α-SiAlON composite of the order of &gt;98% theoretical density, having high hardness and high fracture toughness. The dense β-Si 3 N 4 -α-SiAlON composite will be useful for low temperature applications as wear parts like bearing and roller materials and particularly for grinding and milling operations like grinding balls.

FIELD OF THE INVENTION

The present invention relates to a synergistic composition useful for the preparation of dense neodymium stabilised β-Si₃N₄-α-SiAlON composite. Dense β-Si₃N₄-α-SiAlON composite finds usage in low temperature applications such as wear parts like bearing and roller materials and particularly for grinding and milling operations like grinding balls.

BACKGROUND OF THE INVENTION

It is known in the art to hot pressing green mixtures of Si₃N₄, AlN, Al₂O₃ and Nd₂O₃, at a temperature in the range of 1550° to 1750° C., and at a pressure of about 20 MPa (Wang et al. in Mater. Res. Soc. Symp. Proc., Vol. 287, 1993, pp. 387–392 titled “Formation and densification of R-α′ SiAlONs (R=Nd,Sm,Gd,Dy,Er,Yb)”). Shen et al. (J. Am. Ceram. Soc., Vol. 79, No. 3, 1996, pp. 721–32 titled “Homogeneity region and thermal stability of neodymium-doped α SiAlON ceramics”) teach hot pressing to fabricate the material as stated above.

O'Reilly et al. (Mater. Res. Soc. Symp. Proc., Vol. 287, 1993, pp. 393–398 titled “Parameters affecting pressureless sintering of α′ SiAlONs with lanthanide modifying cations”) discloses that green mixture containing similar starting materials as above were pressureless sintered but yielded only 50% α-SiAlON in the sintered product. Kall et al. (J. Eur. Ceram. Soc., Vol. 6, 1990, pp. 191–27, titled “Sialon ceramics made with mixtures of Y₂O₃—Nd₂O₃ as sintering aids”) discloses that green mixtures were pressureless sintered above 1825° C. Although the high temperature firing could produce fully sintered material, the pressureless sintering at 1750° C. could only produce up to 96% of theoretical density even when α-SiAlON is completely absent.

The major drawbacks of the above noted hitherto known processes are that these involve selection of a composition that requires hot pressing for full densification, which is evidently expensive. It is also difficult to manufacture a complex-shaped material and also failed to produce high densification under pressure less sintering method.

Thus, there is a need to provide a composition for preparation of dense neodymium stabilised β-Si₃N₄-α-SiAlON composite, which overcome the above disadvantages.

OBJECTS OF THE INVENTION

The main object of the present invention is to provide a synergistic composition useful for the preparation of dense neodymium stabilised β-Si₃N₄- α-SiAlON composite, which obviates the drawbacks of the hitherto known prior art.

Another object of the present invention is to provide a synergistic composition, useful for the preparation of dense neodymium stabilised β-Si₃N₄-α-SiAlON composite, wherein the composition consists of a synergistic mixture of Si₃N₄, Al₂O₃, AlN, SiO₂ and Nd₂O₃.

Still another object of the present invention is to provide a synergistic composition, useful for the preparation of dense neodymium stabilised β-Si₃N₄-α-SiAlON composite, wherein the composition displays easier densification under normal sintering conditions.

Yet another object of the present invention is to provide a synergistic composition, wherein a cheaper additive oxide such as neodymium oxide in comparison to other oxides such as dysprosium oxide or ytterbium oxide, makes the composition economic.

SUMMARY OF THE INVENTION

The present invention provides a synergistic composition useful for the preparation of dense neodymium stabilised β-Si₃N₄-α-SiAlON composite, wherein the composition consists of a synergistic mixture of Si₃N₄, Al₂O₃, AlN, SiO₂ and Nd₂O₃.

Accordingly, the present invention provides a synergistic composition useful for the preparation of dense neodymium stabilised β-Si₃N₄-α-SiAlON composite, which comprises:

-   Si₃N₄: 49 to 63 mole %, -   Al₂O₃: 2.5 to 3.5 mole %, -   AlN: 29 to 43 mole %, -   SiO₂: 2.1 to 2.9 mole %, and -   Nd₂O₃: 3.1 to 4.9 mole %.

In an embodiment of the present invention, the Si₃N₄ contains oxygen less than 1 weight %.

In another embodiment of the present invention, the Al₂O₃ is of purity greater than 98%.

In still another embodiment of the present invention, the AlN contains oxygen less than 2.5 weight %.

In yet another embodiment of the present invention, the SiO₂ is of purity greater than 98%

In still yet another embodiment of the present invention, the Nd₂O₃ is of purity greater than 98%.

The composition of the present invention is not a mere admixture but a synergistic mixture having properties which are distinct and different from the mere aggregation of the properties of the individual ingredients. There is no chemical reaction in the said synergistic mixture.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a synergistic composition useful for the preparation of dense neodymium stabilised β-Si₃N₄-α-SiAlON composite. The composition comprises essentially of:

-   Si₃N₄: 49 to 63 mole %, -   Al₂O₃: 2.5 to 3.5 mole %, -   AlN: 29 to 43 mole %, -   SiO₂: 2.1 to 2.9 mole %, and -   Nd₂O₃: 3.1 to 4.9 mole %.

The Si₃N₄ contains oxygen less than 1 weight %. The Al₂O₃ is preferably of purity greater than 98%. The AlN preferably contains oxygen less than 2.5 weight %. The SiO₂ is also preferably of purity greater than 98%. The Nd₂O₃ is of purity greater than 98%.

The novelty of the present invention resides in providing a synergistic composition useful for the preparation of dense neodymium stabilised β-Si₃N₄-α-SiAlON composite, wherein the product obtained using the selected compositional zone from the system Si₃N₄—Al₂O₃.AlN—Nd₂O₃.9AlN—SiO₂ exhibits α-SiAlON as single crystalline phase with excellent sinterability and possesses a final density value of not less than 98% of theoretical in the temperature range >1750° C. This has been made possible by providing a composition which consists of a synergistic mixture of Si₃N₄, Al₂O₃, AlN, SiO₂ and Nd₂O₃.

Our copending application Ser. No. 10/974,016 describes and claims a process for the manufacture of dense neodymium stabilised β-Si₃N₄-α-SiAlON composite from the synergistic composition of the present invention, which comprises preparing a homogeneous mixture of the composition, passing the powder through 100 mesh, pressing the powder to form green compacts, sintering the green compacts at a temperature in the range of 1700° to 1900° C. in nitrogen atmosphere.

The sintering is found to be enhanced in β-Si₃N₄-α-SiAlON compositions when selected from the system Si₃N₄—Al₂O₃.AlN—Nd₂O₃.9AlN—SiO₂. It is believed that the mechanism is as follows: In general, the sintering of the α-SiAlON materials are difficult primarily due to the presence of some secondary intermediate crystalline phases. In cases of both yttrium- as well as some rare earth-doped compositions, the melilite phase, M₂O₃.Si₃N₄ (M=Y, Yb, Dy, Sm, Nd, etc.) often containing aluminium in solid solution, occur frequently together with α-SiAlON in the intermediate sintering temperature range. The phase absorbs large amount of the doping element and becomes competitive for the volume fraction of the liquid phase present thereby hindering densification and the precipitation of α-SiAlON as well. The final densification of the material therefore becomes dependent on the dissociation temperatures of the melilite which promotes the amount of the liquid phase once again at high temperature so that the sintering proceeds. The extent of the melilite phase formation is favoured when the starting composition is taken in the nitrogen rich side of the compositional zone. It is believed that the introduction of SiO₂ in the starting composition disfavours the formation of the nitrogen rich crystalline phases like melilite etc. and also favours the formation of a larger amount of liquid during sintering thereby promoting an improved densification at comparatively lower temperature with respect to the compositions without SiO₂.

The following examples are illustrative of the use of the synergistic composition of the present invention and therefore, should not be construed to limit the scope of the invention in any manner.

EXAMPLE 1

A composition containing Si₃N₄— 53.3 mole %, Al₂O₃— 2.6 mole %, AlN— 37.7 mole %, Nd₂O₃— 4 mole % and SiO₂— 2.4 mole %, was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1750° C. for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 15.72%, the firing weight loss was 2.14%. The fired density was 98.06% of the theoretical value. The β-Si₃N₄:α-SiAlON phase ratio is lesser than 1:5.

EXAMPLE 2

A composition containing Si₃N₄— 53.3 mole %, Al₂O₃— 2.6 mole %, AlN— 37.7 mole %, Nd₂O₃— 4 mole % and SiO₂— 2.4 mole %, was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1800° C. for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 16.11%, the firing weight loss was 2.2%. The fired density was 98.64% of the theoretical value. The β-Si₃N₄:α-SiAlON phase ratio is lesser than 1:5.

EXAMPLE 3

A composition containing Si₃N₄— 53.3 mole %, Al₂O₃— 2.6 mole %, AlN— 37.7 mole %, Nd₂O₃— 4 mole % and SiO₂— 2.4 mole %, was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1825° C. for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 16.13%, the firing weight loss was 2.28%. The fired density was 98.76% of the theoretical value. The β-Si₃N₄:α-SiAlON phase ratio is lesser than 1:5.

EXAMPLE 4

A composition containing Si₃N₄— 53.3 mole %, Al₂O₃— 2.6 mole %, AlN— 37.7 mole %, Nd₂O₃— 4 mole % and SiO₂— 2.4 mole %, was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1850° C. for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 16.32%, the firing weight loss was 2.29%. The fired density was 98.84% of the theoretical value. The hardness of the final product is 18.6 GPa. The fracture toughness of the final product is 4.7 MPa.m^(1/2). The β-Si₃N₄:α-SiAlON phase ratio is lesser than 1:5.

EXAMPLE 5

A composition containing Si₃N₄— 53.3 mole %, Al₂O₃— 2.6 mole %, AlN— 37.7 mole %, Nd₂O₃— 4 mole % and SiO₂— 2.4 mole %, was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1900° C. for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 16.19%, the firing weight loss was 2.71%. The fired density was 98.81% of the theoretical value. The β-Si₃N₄:α-SiAlON phase ratio is lesser than 1:5.

EXAMPLE 6

A composition containing Si₃N₄— 49.06 mole %, Al₂O₃— 2.75 mole %, AlN— 41.50 mole %, Nd₂O₃— 4.50 mole % and SiO₂— 2.19 mole %, was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1750° C. for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 16.14%, the firing weight loss was 1.98%. The fired density was 98.22% of the theoretical value. The β-Si₃N₄:α-SiAlON phase ratio is greater than 3:1.

EXAMPLE 7

A composition containing Si₃N₄— 49.06 mole %, Al₂O₃— 2.75 mole %, AlN— 41.50 mole %, Nd₂O₃— 4.50 mole % and SiO₂— 2.19 mole %, was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1800° C. for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 16.29%, the firing weight loss was 1.98%. The fired density was 98.39% of the theoretical value. The β-Si₃N₄:α-SiAlON phase ratio is greater than 3:1.

EXAMPLE 8

A composition containing Si₃N₄— 49.06 mole %, Al₂O₃— 2.75 mole %, AlN— 41.50 mole %, Nd₂O₃— 4.50 mole % and SiO₂— 2.19 mole %, was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1850° C. for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 16.38%, the firing weight loss was 2.09%. The fired density was 98.44% of the theoretical value. The β-Si₃N₄:α-SiAlON phase ratio is greater than 3:1.

EXAMPLE 9

A composition containing Si₃N₄— 49.06 mole %, Al₂O₃— 2.75 mole %, AlN— 41.50 mole %, Nd₂O₃— 4.50 mole % and SiO₂— 2.19 mole %, was attrition milled for 3 h, dried, cold pressed under isostatic pressure and was fired at 1900° C. for 2 h in a nitrogen gas atmosphere under a gas pressure of 1 MPa. The linear shrinkage was 16.18%, the firing weight loss was 2.22%. The fired density was 98.41% of the theoretical value. The β-Si₃N₄:α-SiAlON phase ratio is greater than 3:1.

The Main Advantages of the Present Invention are:

-   1. Provides a synergistic composition, useful for the preparation of     dense neodymium stabilised β-Si₃N₄-α-SiAlON composite. -   2. Provides a synergistic composition, useful for the preparation of     dense neodymium stabilised β-Si₃N₄-α-SiAlON composite, wherein the     composition displays easier densification under normal sintering     conditions. -   3. Provides a synergistic composition, wherein a cheaper additive     oxide such as neodymium oxide in comparison to other oxides such as     dysprosium oxide or ytterbium oxide, makes the composition economic. -   4. Provides a synergistic composition which does not require hot     pressing thereby providing a cost effective method for the     preparation of β-Si₃N₄-α-SiAlON material. 

1. A composition for preparation of dense neodymium stabilised β-Si₃N₄-α-SiAlON composite, which comprises: Si₃N₄: 49 to 63 mole %, Al₂O₃: 2.5 to 3.5 mole %, AlN: 29 to 43 mole %, SiO₂: 2.1 to 2.9 mole %, and Nd₂O₃: 3.1 to 4.9 mole %.
 2. A composition as claimed in claim 1, wherein the Si₃N₄ contains less than 1 weight % oxygen.
 3. A composition as claimed in claim 1, wherein the Al₂O₃ has purity greater than 98 weight %.
 4. A composition as claimed in claim 1, wherein the AlN contains less than 2.5 weight % oxygen.
 5. A composition as claimed in claim 1, wherein the SiO₂ has purity greater than 98 weight %.
 6. A composition as claimed in claim 1, the Nd₂O₃ has purity greater than 98 weight %. 